feat: add cluster token encryption

backend/internal/encryption/cipher.go

@@ -0,0 +1,79 @@
+package encryption
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"encoding/base64"
+	"fmt"
+	"io"
+	"strings"
+)
+
+const (
+	currentVersion byte = 1
+	keySize             = 32
+)
+
+type Cipher struct {
+	gcm cipher.AEAD
+}
+
+func New(masterKey []byte) (Cipher, error) {
+	if len(masterKey) != keySize {
+		return Cipher{}, fmt.Errorf("master key must be %d bytes", keySize)
+	}
+
+	block, err := aes.NewCipher(masterKey)
+	if err != nil {
+		return Cipher{}, err
+	}
+
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return Cipher{}, err
+	}
+
+	return Cipher{gcm: gcm}, nil
+}
+
+func NewFromBase64(encodedKey string) (Cipher, error) {
+	encodedKey = strings.TrimSpace(encodedKey)
+	if encodedKey == "" {
+		return Cipher{}, fmt.Errorf("MASTER_KEY_BASE64 is required")
+	}
+
+	key, err := base64.StdEncoding.DecodeString(encodedKey)
+	if err != nil {
+		return Cipher{}, fmt.Errorf("MASTER_KEY_BASE64 must be base64 encoded: %w", err)
+	}
+
+	return New(key)
+}
+
+func (c Cipher) Encrypt(plaintext []byte) ([]byte, error) {
+	nonce := make([]byte, c.gcm.NonceSize())
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return nil, err
+	}
+
+	ciphertext := make([]byte, 0, 1+len(nonce)+len(plaintext)+c.gcm.Overhead())
+	ciphertext = append(ciphertext, currentVersion)
+	ciphertext = append(ciphertext, nonce...)
+	ciphertext = c.gcm.Seal(ciphertext, nonce, plaintext, nil)
+	return ciphertext, nil
+}
+
+func (c Cipher) Decrypt(ciphertext []byte) ([]byte, error) {
+	nonceSize := c.gcm.NonceSize()
+	if len(ciphertext) < 1+nonceSize+c.gcm.Overhead() {
+		return nil, fmt.Errorf("ciphertext is too short")
+	}
+	if ciphertext[0] != currentVersion {
+		return nil, fmt.Errorf("unsupported ciphertext version")
+	}
+
+	nonce := ciphertext[1 : 1+nonceSize]
+	encrypted := ciphertext[1+nonceSize:]
+	return c.gcm.Open(nil, nonce, encrypted, nil)
+}